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I have a 70-year-old female patient with brief visual hallucinations around 3 or 4 am only. There are no other symptoms to suggest narcolepsy or psychosis. Do hypnagogic hallucinations only occur when falling asleep?
Alexandra Thomson, MD, Kelowna, BC

Without more details, it’s hard to know the diagnosis of your patient. As you mention, hypnagogic hallucinations occur during the transition from wakefulness to sleep and are often seen in narcolepsy. Narcoleptics can also experience hypnopompic hallucinations upon awakening. Visual hallucinations occur in a number of other conditions. Occasional hypnagogic or hypnopompic hallucinations aren't rare in normal subjects without narcolepsy either.

Visual hallucinations can accompany psychosis but auditory hallucinations are much more common in that case. Encephalopathic states such as delirium tremens are also often accompanied by hallucinations (see the 1945 film The Lost Weekend for a vivid cinematic example). Many intoxications may cause people to hallucinate (check out Lucy in the sky with diamonds for a musical description).

Charles Bonnet syndrome is a fairly common condition that usually affects older patients with significant visual loss. They experience complex visual hallucinations and are generally quite aware that these are not real. The hallucinations are often smaller than life and sometimes cartoon-like.

Visual hallucinations are a hallmark of Lewy body dementia. Animals or children are common sights. Some of these patients are aware of the unreality of the creatures and aren't bothered by them while others may be very distressed.

Visual hallucinations are occasionally a part of a complex partial seizure. They are also common in migraine aura but are rarely described as well-formed objects.

Lastly, as we move from the horses to the zebras, no neurologist’s differential diagnosis would be complete without a mention of the rare peduncular hallucinosis due to brainstem or thalamic lesions.

Reference

1. Kirk A. Music of the hemispheres. Canadian Journal of Neurological Sciences 2009;36(4):531-3.